1. Field of the Invention
This invention relates to a process for producing acetaldehyde, and more particularly it relates to a process for producing acetaldehyde by oxygen-oxidizing ethylene in the presence of a metal complex catalyst.
2. Description of the Prior Art
Acetic acid and aldehydes as basic chemicals for petrochemical industry have been produced by oxidation reactions of the corresponding raw materials. Such oxidation reactions occupy an important position in reaction processes in the field of petrochemical industry. These oxidation reactions have so far been carried out at high temperatures and high pressures, but improvement of reaction selectivity and yield is becoming important problem to be settled.
Acetaldehyde is an important, basic material for production of many organic substances, and its derivatives including many compounds such as acetic acid, acetic esters, etc. As for processes for producing acetaldehyde, the acetylene hydration process, ethanol dehydrogenation process and the ethylene direct oxidation process have been practically employed. However, among them, acetylene the hydration process and ethanol dehydrogenation process are no longer used as commercial processes, since a larger amount of byproducts are formed in a large amount due to their severe reaction conditions. In contrast to these processes, a so-called Wacker's process wherein ethylene is used as raw material and palladium chloride (Pd(2)Cl.sub.2)-cupric chloride (Cu(2)Cl.sub.2) is used as catalyst, has been noted as the process for producing acetaldehyde under mild reaction conditions, and has come to be the main process among current acetaldehyde production processes.
According to the process, a composite catalyst obtained by dissolving Pd(2)Cl.sub.2 and Cu(2)Cl.sub.2 as catalysts in a hydrochloric acid solution (pH: 0.about.2) is employed. Ethylene is first oxidized with divalent palladium (Pd(2) and water H.sub.2 O to form acetaldehyde (CH.sub.3 CHO). The reaction is expressed by the following equation wherein water participates in the reaction: EQU CH.sub.2 =CH.sub.2 +Pd(2)Cl.sub.2 +H.sub.2 O.fwdarw.CH.sub.3 CHO+Pd(0).dwnarw.+2HCl (1)
As seen from the above reaction equation, Pd(2) is reduced to metal palladium Pd(0) which precipitates. This is prevented by making Cu(2)Cl.sub.2 coexistent in a large amount, and at the same time, Pd(0) is oxidized into Pd(2) for regeneration according to the following equation: EQU Pd(0)+2Cu(2)Cl.sub.2 .fwdarw.Pd(2)Cl.sub.2 +2Cu(1)Cl (2)
Further, slightly soluble Cu(1)Cl byproduced at that time is oxygen-oxidized in the copresence of HCl and returned to Cu(2)Cl.sub.2 according to the following equation: EQU 2Cu(1)Cl+1/2O.sub.2 +2HCl.fwdarw.2CuCl.sub.2 +H.sub.2 O (3)
As described above, by employing a redox system of Pd(2)/Pd(0) and Cu(2)/Cu(1), continuous oxidation of ethylene is rendered possible. However, according to this process, the oxygen molecule is not directly reacted with ethylene as described above, but since a complicated oxidation-reduction reactions of Pd(2)/Pd(0)-Cu(2)/Cu(1) system is utilized, these reactions constitute a reaction rate-determining step. Further, since slightly soluble Pd(0) and Cu(1)Cl are formed midway during the reactions, a concentrated HCl aqueous solution having a high concentration (PH: 0.about.2) must be used; hence it is necessary to select a corrosion-resistant material. Further, since oxygen has a low solubility in water, it is necessary for accelerating the reaction by increasing its amount dissolved, to carry out the reaction under pressure and heating conditions such as 10 Kg/Cm.sup.2 and 100.degree. C. Furthermore, when dissolved oxygen in excess is released into the gas phase, ethylene mixes with oxygen, resulting in a possibility of troubles such as explosion; hence a countermeasure to safety is required.
The object of the present invention is to provide a process for producing acetaldehyde by oxygen-oxidizing ethylene at lower temperatures and lower pressures, selectively and with a high yield.